I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing handoff on the forward link in wireless communication systems that use multi-carrier modulation (e.g., OFDM) for the forward link and CDMA for the reverse link.
II. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources. Such systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), some other multiple access technique, or a combination thereof. CDMA systems may also be designed to implement known CDMA standards such as cdma2000, IS-856, IS-95, W-CDMA, and others.
In a direct sequence (DS) CDMA system, a narrowband signal is spread over the entire system bandwidth in the time domain with a spreading sequence. Some examples of such DS-CDMA systems include those that implement IS-2000, IS-95, and W-CDMA standards. The spreading sequence may be a pseudo-random number (PN) sequence (e.g., for IS-95 and IS-2000) or a scrambling sequence (e.g., for W-CDMA). A DS-CDMA system provides certain advantages such as ease of supporting multiple access, narrow-band rejection, and so on.
Orthogonal frequency division multiplexing (OFDM) effectively partitions the system bandwidth into a number of (N) orthogonal subbands, which are also often referred to as tones, frequency bins, and frequency subchannels. In each time interval that is dependent on the bandwidth of each subband, a modulation symbol may be transmitted on each of the N subbands. OFDM may be used to combat inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. ISI is caused by frequency selective fading in a multipath channel. To combat ISI, a portion of each OFDM symbol is repeated prior to transmission, as is known in the art.
For various reasons, it may be advantageous to use one modulation technique for one communication link (e.g., OFDM for the forward link) and another modulation technique for the complementary communication link (e.g., CDMA for the reverse link). However, the use of different modulation techniques may complicate certain system operation, such as handoff of terminals between base stations in the systems.
There is therefore a need in the art for techniques to perform handoff on the forward link in hybrid wireless communication systems.